1. Field of the Invention
This invention relates to a circuit breaker wherein an overvoltage suppression portion is connected in parallel with a circuit breaker portion.
2. Description of the Prior Art
In general in power transmission systems of 500 KV or more, a resistance closing system is adopted. In such a system, particularly to suppress the closing overvoltage of the circuit breaker on closing, a closing resistor is arranged electrically in parallel with the main contacts breaker portion of the circuit-breaker, and this closing resistor is closed in advance of the main contacts circuit-breaker portion. In this case, resistance switching contacts that switch the closing resistor are operated by a link mechanism that is mechanically linked with a main contacts circuit-breaking portion.
With the rapid technical progress of recent years, circuit-breakers fitted with closing resistors, in particular SF.sub.6 gas circuit breakers, have been developed which are of much smaller size, with a greatly reduced number of circuit-breaking points. Two-point break circuit breakers have been developed for use even in 500 KV systems. The value of the closing resistance which suppresses the closing overvoltage is determined by the coordination of insulation level of the power supply system, and the system voltage, system surge impedance and length of the supply lines, and is not much affected by the short-circuit capacity.
Consequently, although, as mentioned above, circuit-breaker performance has been improved and the number of break-points have been considerably reduced, with considerable reduction in the interior space of the circuit-breaker, the present situation is that the size of the closing resistor has not been altered. In the effort to achieve compact circuit breakers, the efficient arrangement of the resistor switch contacts and of the closing resistor has therefore become a major problem.
FIG. 1 shows the construction of a typical convention 4-point-break circuit breaker. FIGS. 2 and 3 show the construction and arrangement of the main contacts, closing resistor and resistance switching contacts of a conventional circuit-breaker fitted with a closing resistor. Such a circuit breaker is constructed as follows. Four point circuit breaker units 2a, 2b, 2c and 2d are connected in series inside the tubular tank 1. Both ends thereof are connected to the outside through bushings 3a and 3b. Units 2a, 2b and 2c, 2d correspond to drive parts 4a and 4b, respectively. The circuit-breaker units 2a, 2b, 2c and 2d are respectively provided with a main contact circuit breaker portion 6 and an overvoltage suppression portion 40, consisting of resistance switching contacts 5 and closing resistor 14 connected in series. Resistance switching contacts 5 switch the current that flows in the closing resistor 14, and include a fixed contact 10 provided with a wipe spring 9 and a movable contact 13 that can be advanced or retracted by means of a drive mechanism 12 which includes a lever 11 etc., these contacts being arranged in the interelectrode space 8 formed by an insulating tube 7. The movable contact 13 can be advanced or retracted through a through-hole of the closing resistor 14, which is of cylindrical shape and is provided for suppressing the switching overvoltage of the main contact circuit breaker portion 6. The extreme end of the movable contact 13 is covered by a shield 15 for relaxing or controlling the electric field. The end of the fixed contact 10 is likewise covered by a shield 16.
The main contact circuit breaker portion 6 consists of a fixed contact 20 constructed of a fixed electrode 19 and a main fixed contactor 18 covered by a shield 17 in tank 1, and a movable contact 25 constructed of a movable electrode 24 and a main movable contactor 23 fixed to a puffer cylinder 22 that is slidably fitted onto the outside of a puffer piston 21 fixed to a support part, not shown. The movable contact 25 can be advanced or retracted by means of a drive mechanism 27 formed by a lever 26, and is equipped with a nozzle 28, made of an insulator, at its tip.
The lever 11 of the resistance switching contacts 5 and the lever 26 of the main contacts circuit breaker portion 6 are mechanically linked by a shaft 29 so that they operate in tandem. However, the transmission of drive force by the shaft 29 imparts a torsional force to this shaft 29 which appreciably lowers its fatigue limit, making it necessary to use expensive materials. Furthermore, since the resistance switching contacts 5 and main contacts circuit-breaker 6 are linked by the shaft 29, the resistance switching contacts 5 are arranged laterally of and in parallel with the main contacts 6.
From the viewpoint of design of the electric field, the arrangement of the main contacts circuit-breaker portion 6 and the resistance switching contacts 5 is generally determined by the respective ground insulating distances (i.e. the shortest distance between the innerface of tank 1 and the outerface of main contacts circuit-breaker portion 6, and between the innerface of tank 1 and the outerface of resistance switching contacts 5) and the creeping stress of an insulating cylinder 30 for supporting main contacts circuit-breaker portion 6. Therefore, the diameter of tank 1 becomes fairly large if insulating cylinder 30 is mounted with its axial center line coinciding with the center line of tank 1, so that the main contacts circuit-breaker portion 6 is arranged eccentrically in tank 1. In this case, first the ground distance S.sub.1, between resistance switching contacts 5 and tank 1 is set such that the dimension S.sub.1, becomes the shortest dimension and falls within the scope of allowable values of electric field. Secondly, the dimension S.sub.2 is set within the scope of allowable values of the creeping stress because of creeping stress of insulating cylinder 30 is weaker than the electric stress of the gas gap forming in tank 1. The creeping stress becomes high if the dimension S.sub.2 is too short, whereupon as a result of the creeping stress, insulating cylinder 30 will be destroyed.
The diameter of tank 1 is finally determined so as to be able to keep the dimensions within the scope desired above.
Thus, the circle 31 including main contact circuit breaker portion 6 and resistance switching contacts 5 is formed as shown in FIG. 2.
In case that resistance switching contacts portion 5 is arranged directly above the circuit breaker portion 6 (not shown), the size of tank 1 also becomes large by the vertical extension of contacts portion 5 above the main contact circuit breaker portion 6. (See Japanese Patent Disclosure (Kokai) No. 50-45280).
In case that resistance switching contacts portion 5 is mounted on and horizontally in parallel with main contact circuit-breaker portion 6, the size of tank 1 becomes large by the resultant eccentric quantity of tank 1, i.e. the quantity shifted by the arrangement of resistance switching contacts portion 5. (See Japanese Patent Disclosure (Kokai) No. 53-35971).